It seems like you are using several of your variable names for more than one thing. That may confuse Mathematica.
Can you see if you can understand this idea and perhaps adapt it to what you want to do?
Clear[t];(*Make sure t has no assigned value*)
V[t_] := 10 (1 - Exp[-0.1 t]);(*Define V function of t*)
dV = D[V[t], t];(*Define derivative of V at any t*)
v = 0;(*Start the iteration value with v=0*)
(*Now create a list of {time,v} values where t is replaced with time in dV*)
points = Table[{time, v = v + dV /. t -> time}, {time, 0., 10, tau}]
That tries to use the function V[t] for only that purpose, the derivative dV for only that purpose, the iteration time for only that purpose and the iteration value for only that purpose.
Then you can plot the result using this
ListPlot[points]
If I have misunderstood and you can explain then perhaps I will be able to correct this for you